Using latest Vaadin (8.11.0) I’ve came up on strange issue - too deep recursion during Inputmask initialization → within method $.extend.
I’ve cloned your addon updating to latest Inputmask (5.0) - no success.
After investigation I found that GWT is changing Integer to Object with complex prototype during options create. Extend method called with deep copy was falling into recursion.
I’ve changed GwtMaskOptions all Integers to ints and it fixed the issue.